This invention relates generally to magnetostrictive vibrators and ferrites therefor and more particularly to the production of ferrites comprising sintered magnetite for magnetostrictive vibrators.
Some of the properties of sintered ferrite structures required for use as magnetostrictive vibrators are, according to our knowledge, as follows:
(a) High magnetostrictive properties, i.e., high electromechanical coupling coefficient k; PA1 (b) High mechanical strength, i.e., high transverse rupture strength and high vibrational fatigue strength; and PA1 (c) High electrical resistance.
In general, ferrites for magnetostrictive vibrators have the advantageous feature of higher resistivity or specific resistance even at frequencies in the ultrasonic region and, therefore, less eddy current loss whereby the electroacoustic efficiency of such magnetostrictive vibrators is improved, in comparison with metal magnetostrictive vibrators. For this reason, these ferrites have been widely used as vibrators of ultrasonic equipment, the principal examples of which are ultrasonic washing machines and ultrasonic fabricating machines and tools.
The ferrites used at present for magnetostrictive vibrators are all materials having Ni-Cu-Co ferrite as their principal constituent, but these materials are of complicated composition and are accompanied by the problem of high production cost due to high cost of the starting materials.
On one hand, magnetites have been known as exhibiting magnetostrictive property.
For example, saturated static magnetostriction of a magnetite is of the order of +40.times.10.sup.-6 while that of a ferrite is of the order of -27.times.10.sup.-6.
Of the spinel-type magnetostrictive materials, only the static magnetostriction of a magnetite is positive while that of Ni ferrite or Ni-Cu-Co ferrite is negative. A magnetostrictive vibrator having positive magnetostriction is considered to be more advantageously used under high input power as compared with the one having negative magnetostriction, the absolute value of both magnetostrictions being equal. In the case of ordinary uses, however, it matters little whether static magnetostriction is positive or negative but it is important for a magnetostrictive vibrator to have a large absolute value.
High static magnetostriction alone does not satisfy the static magnetostrictive property required for a magnetostrictive vibrator but it is also preferable that such a vibrator should have high static magnetostriction when effective biassing magnetic field is at a level as low as from 5 to 50 Oe which is usually employed.
Magnetites have the advantageous features of not only positive saturated static magnetostriction as well as positive static magnetostriction in a low effective biassing magnetic field but also a larger absolute value in comparison with Ni-Cu-Co ferrite.
In spite of these excellent static magnetostrictive properties, magnetites have not heretofore been used as magnetostrictive materials because a sintered magnetite structure having all the aforementioned properties (a), (b) and (c) required for a magnetostrictive material could not be obtained therefrom.
In connection with the above properties (a), (b) and (c), a sintered structure should have high density, and suitable grain size and composition while macroscopic inhomogeneity should not be caused in the sintered structure such as pores and cracks.
Hereinbelow, these requirements for a sintered structure will be set forth in more detail.